The invention relates generally to cryogenic medical devices. More particularly, the invention relates to a cryogenic medical device for delivering a pressurized sub-cooled, mix-phased cryogen to a cryoprobe.
With the strong movement in the medical community toward the use of minimally invasive therapies, ablation therapies are becoming more prevalent. Cryotherapy (or cryoablation) is a minimally invasive method of treating disease by freezing an area to ablate a target tissue. Cryotherapy provides an alternative to radical surgery, radiation therapy, chemotherapy, and hyperthermal ablation. Advantages of ablation therapies relative to these conventional therapies include, for example, precise eradication of targeted tissue, decreased hospitalization time, limited postoperative morbidities, shortened return interval to activities of daily living, reduced severity and incidence of side effects, and reduced overall treatment cost.
Cryotherapy is currently used to treat numerous disease states including but not limited to: benign and cancerous tumors of the prostate, kidney, liver, pancreas, bone, and skin, as well as cardiovascular disease, retinal detachment, pain management, and other illness/disease states. In many applications, it is desirable to be able to selectively freeze a very small area to a very low temperature without affecting the temperature of surrounding tissues and organs.
Cryogens that have been used for ablation procedures include liquid nitrogen (LN2), critical nitrogen (CN), supercritical nitrogen (SCN), nitrous oxide (NO), argon gas (Ar), and carbon dioxide (CO2). Current systems and devices have focused on delivery of the liquid cryogen through the use of low to moderate pressure (15-450 psi) on the entire system, piston/bellows compression to drive fluid movement, creation of critical or supercritical states through heat and pressurization (500-1200 psi), or alternatively, the use of cryogen gases such as nitrous oxide, carbon dioxide, and argon under extremely high pressures, e.g., 3,000-6,000 psi in Joule-Thomson based systems. Each of these systems has significant drawbacks.
In the case of high pressure, gas cryogen Joule-Thomson based systems, ineffective cooling and limited ability to drive ablative temperatures (e.g., −40° C.) into target tissues is a significant inherent weakness. Further, Joule-Thomson systems rely on the use of costly rare gases such as argon and nitrous oxide, delivered under high pressures, which limits the use of these systems.
Liquid systems, on the other hand, provide colder temperatures and greater freezing (heat extraction) capacity, but are slow to achieve a target temperature and often result in over-freezing of tissue. This results in unwanted collateral damage to surrounding tissue. Finally, systems utilizing cryogens in a critical or supercritical state, while providing a more powerful and quicker freeze, require complicated device architecture and larger cryogen reservoirs, and face challenges in the ability to run multiple probes in complex sequences.